@Ed Obermeier hit the mark - skiboat engines need torque down low, which brings to mind the old hot rod axiom, "there's no replacement for displacement"... You can turbocharge an engine, but it will typically make power at higher rpms only. Supercharging (pulley or gear-driven forced induction) will help a smaller displacement engine make big power in the low-rpm range, but with added expense and complication, I don't see it as a viable substitute for a big-cube V8. Wankels and two-strokes are characteristically high-rpm power, lower torque engines as well. You can re-engineer them to bring on the power lower in the range, but it's against the nature of their design - I guess that's why we still see 99% of our slalom boats driven by big V8s.
Refining the current V8 engines' management systems, as well as better combustion chamber design are the short-term future for better economy & power (where we need it, i.e., in the low rpm range), IMO. Turbodiesel power seems like a possible alternative too, if the cost of diesel oil becomes significantly cheaper (and cleaning up their emissions too).
Another aspect is the weight. Sure, lighter is better, but up to a point: slalom boats benefit from the inertia to keep the pull constant.
Great post!